The present invention generally pertains to the field of cathode ray tubes. More particularly, embodiments of the present invention are related to regulating beam current in a cathode ray tube beam using autobias.
A conventional cathode ray tube (CRT) creates an image on a phosphor coated display screen by focusing a beam of electrons onto the screen and thus making the phosphors emit visible light. The electron beam is typically formed by first causing a cathode inside the vacuum of the CRT to emit electrons by applying a differential voltage between the cathode and a grid. The CRT has one or more anodes to attract, focus, and accelerate the electrons. Thus, the anodes may collimate the electrons into a tight beam hitting a spot of the display screen. By creating currents in coils outside the CRT, a magnetic field is created within the CRT to direct the electron beam at any desired spot of the display screen.
As the phosphors in a CRT respond to the intensity of the beam current, the brightness of the picture will be a function of the beam current. However, the beam current does not stay constant for a given cathode voltage over its lifetime. Hence, the picture intensity will typically change even though the same voltage is applied to the cathode. Thus, it is desirable to have a means to compensate to keep the CRT picture uniform over time.
Compensation systems have been proposed to reduce this problem. Some techniques are suitable for a DC coupled system, but not for an AC coupled system. Such a DC coupled system typically requires significant power and slows the response of the system relative to an AC coupled system.
Thus, a need exists for a technique for compensating for changes in picture intensity in a CRT due to aging on the CRT. A further need exists for a technique that does not consume considerable power. A still further need exists for a technique that does not change the time response of the system when varying the bias voltage. A still further need exists for a technique to be used in an AC coupled system.
The present invention provides a method and system for regulating beam current in a cathode ray tube. Embodiments consume less power than some conventional techniques. Embodiments do not change system response time when varying the bias voltage as do some conventional techniques. Embodiments are suitable for a system in which the CRT is AC coupled to driver circuitry.
A system and method for regulating beam current in a cathode ray tube are disclosed. In a system embodiment, the system may comprise test signal timing and pulse generation logic for outputting test signals operable to stimulate a cathode ray tube at two voltage levels. The system may have a circuit that inputs the test signal timing and determines correction signals based on signals derived from the cathode in response to stimulating the cathode ray tube at the two voltage levels.
In various embodiments, the correction signals may comprise a gain correction signal, a bias correction signal or both. The gain correction signal may modify a signal output to a gain driver that may be AC coupled to the cathode. The bias correction signal may be output to a clamping circuit coupled to the cathode.
A method embodiment comprises generating a test signal comprising first and second pulses operable to create respective first and second current levels of a cathode. This embodiment compares a first reference signal to a first signal derived from the cathode in response to the first pulse applied to the cathode. The embodiment modifies a bias signal for the cathode with a first compensating signal based on this comparison. This embodiment compares a second reference signal to a second signal derived from the cathode in response to the second pulse applied to the cathode. The embodiment modifies a gain signal for the cathode with a second compensating signal based on this second comparison.
These and other advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.